1. Field of the Invention
The present invention relates to a method and apparatus for processing corrugated paperboard, and more particularly, to a method and apparatus for producing a heated fluid film between a heating plate a paperboard web.
2. Description of the Prior Art
The manufacturing of double face corrugated paperboard typically begins with an apparatus known as a single facer. A conventional single facer includes an upper corrugating roll and a lower corrugating roll wherein each roll has a plurality of longitudinally extending teeth. The corrugating rolls are rotatably mounted adjacent each other such that the teeth of each roll are in a meshing relationship. A medium web typically passes through a preheater for conditioning and is then fed into the nip point of the upper and lower corrugating rolls wherein the medium web conforms to the contour of the meshing teeth to form flutes in the medium web.
To preheat the medium web, the preheater typically comprises a steam pressurized drum heater having an internal cavity supplied with steam from an external source. The medium web is wrapped around the circumference of the drum and heat from the surface of the drum is transferred to the moving medium web.
A gluing roll, arranged to turn in a bath of starch-based glue, applies glue to the tips of the medium web flutes. A top liner web is simultaneously supplied to a preheater of similar design to the medium web preheater.
Both the top liner web and medium web preheaters depend on conduction for heat transfer to the respective paperboard web. Conduction heat transfer is directly related to the surface area of the paperboard web contacting the preheater and the duration of such contact. In order to provide sufficient heat transfer, the preheaters must therefore define a relatively large surface area and the processing speed of the single facer must be limited. The large surface area required of prior art preheaters substantially increases the overall size of the single facer. In fact, such preheaters are often so large that the preheater must be placed exterior to, and many times behind, the corrugating apparatus. Further, frictional forces opposing the movement of the top liner and medium webs are substantially increased the greater the surface area contacting the webs. Such frictional forces generate tension within the webs, often resulting in web breakage. Prior art attempts to eliminate such problems generated by friction have resulted in complex mechanical arrangements including rotatable preheater drums and variable wrap mechanisms.
The conventional single facer further includes a pressure roll arranged adjacent the lower corrugating roll to apply a nip pressure to the corrugated medium web and the top liner web. The pressure roll and lower corrugating roll are typically heated and the combination of heat and pressure gelatinizes the glue between the medium web and top liner web thereby forming a single face web of corrugated paperboard.
The glue applied to the flutes of the paperboard webs is typically a suspension of raw or uncooked starch in a suitable liquid carrier. In this state, the starch has little or no adhesive qualities. However, at a certain temperature, dependent upon the type of starch utilized and the kind and amount of additives dissolved in the carrier, the starch granules will absorb the liquid of suspension available and swell, causing gelatinization of the suspension. In this state the starch has superior adhesion abilities and will form a good bond between many substrates, including paper. The temperature at which gelatinization occurs for any particular formulation of glue can be easily determined by heating the particular formulation and observing the changes that occur in its viscosity.
After passing over a single face web preheater drum of design similar to the medium web and top liner preheaters, the single face web is next conveyed to a gluing station where the exposed flute tips are covered with a starch-based glue. A bottom liner is typically trained over a preheater in a manner similar to the single face web and then brought into contact with the glued flute tips of the single face web by an apparatus called a double facer to produce a double face web of corrugated paperboard. In order to heat the bottom liner and assist in the gelatinization of the glue between the bottom liner and single face web, the double face web is pressed against and conveyed over an array of heating plates arranged in the direction of web movement. The heating plates define a heating section of the double facer and are typically comprised of cast iron and have central chambers for containing pressurized steam. Inlet and outlet ports in the lower surface of the heating plates provide for the continuous flow of steam.
Double face web travel over the heating plates is conventionally provided by a driven holddown means, usually a continuous holddown belt, in direct contact with the top liner. A series of ballast rollers or the like bear on the inner surface of the holddown belt such that pressure is maintained between the holddown belt and the top liner of the double face web thereby facilitating thermal contact between the web and heating plates.
The conventional double facer apparatus and related method as described above have many inherent disadvantages. For example, since the paperboard is heated by conduction through surface contact between the bottom liner web and top surface of the heating plates, significant frictional forces are developed as the double face web is dragged over the heating plates. Further, if the conventional driven holddown belt is replaced by holddown means having a stationary surface for contacting and holding the web against the heating plates, then additional frictional forces are generated between the top liner and the lower surface of the holddown means as the web is pulled through the double facer by a downstream drawing section. These combined frictional forces result in more horsepower being required to pull the web over the heating plates.
Since the frictional force generated by the web movement is directly proportional to the normal force exerted on the board in the heating section, the pressures in the heating section are deliberately kept much lower than the crush strength of the board in order to avoid even greater horsepower requirements. This, however, results in a reduced heat transfer rate and in turn necessitates a long heating section, typically of forty feet or more. Although the purpose of applying heat to the bottom liner is to raise the temperature of the glue, the glue is actually insulated from the heat source by the bottom liner, resulting in inefficient heat transfer. The prior art process relies on conduction as the primary mode of heat transfer and paper is inherently a poor thermal conductor. In situations where double or triple wall board is being formed, i.e., layers of liners spaced apart by alternating layers of medium, this problem is even more acute since the glue is then insulated by additional layers of liner and medium.
With regard to the quality of the paperboard produced in the conventional process, several common defects in corrugated paperboard are readily traced to the bonding operation in the conventional double facer heating section. For example, warpage of the board is common because of the bonding of a single face web and bottom liner web possessing different moisture levels. After bonding, both webs approach an equilibrium level of moisture content thereby causing differential movement of the two webs, resulting in warpage of the bonded double face web. Additionally, since the boards must be dragged in contacting relationship over the heating plates, some scuffing of the bottom liner will inevitably occur. While this will usually not be serious enough to cause board reject, it does make preprinting of the bottom liner difficult and may necessitate printing of each of the subsequently formed paperboard blanks on an individual basis.
Even with a double facer having a heating section of forty feet or more, the corrugating process speed must be kept fairly low due to poor thermal transfer in the heating section. Additionally, the high frictional forces developed between the web and the heating plates or stationary holddown means result in increased board tensions and a higher frequency of web breakage or tear-outs.
Accordingly, there is a need for a method and apparatus for heating corrugated paperboard which does not generate significant frictional forces against a moving paperboard web and which improves the glue curing times between a medium web and a liner web.
Another problem associated with conventional double facers relates to the process of feeding and threading a web through the heating section in preparation for continuous web processing. The prior art method essentially comprises a "brute force" process of human operators gripping each side of a bottom liner web and then manually pulling the liner web downstream between the heating plates and the holddown means. If a downstream drawing section is utilized for pulling the web through the heating section, then the operators must pull the leading edge of the web through the entire length of the heating section, typically 40 feet or more, and into engagement with the conveying elements of the drawing section. Glued flute tips of the single face web are then manually brought into contact with the bottom liner. Upon start-up of the double facer, the drawing section pulls the bottom liner and single face web through the heating section.
As is readily apparent, the prior art threading process is both difficult and time consuming. Further, the traditional threading process creates significant safety concerns. The operators must manually feed the web through pressure nips defined to receive the paperboard web, resulting in crushing hazards for the hands and fingers of the operators. Additionally, if the double facer has been operating, the process is further complicated by extremely hot components, particularly the surface of the heating plates. Operators must come into close proximity with these hot components during the threading process resulting in the possibility of serious burns being inflicted upon the bodies of the operators.
Accordingly, there is a need for a method and apparatus for threading a web in an safe and efficient manner through a web processing machine. In particular, there is a need for such a method and apparatus for threading a web through the heating section and drawing section of a double facer.